1. Field of the Invention
This invention relates to articles that are capable of absorbing microwave energy. These articles are commonly referred to as susceptors.
2. Discussion of the Prior Art
It is well-known that thin metallic coatings borne on a polymeric sheet will generate heat when placed in a standard microwave oven.
U.S. Pat. No. 4,190,757 discloses a food package composed of a paperboard carton and a lossy microwave energy absorber which becomes hot when exposed to microwave radiation. The absorber is associated in conductive heat transfer relationship with a food product contained in the package and is usually bonded to a structural supporting sheet. The package also includes a shield, e.g., a metal foil sheet adapted to reduce by a controlled amount the direct transmission of microwave energy into the food product. The absorber heats the adjacent surface of the food by conduction to a sufficiently high temperature to provide searing or browning while controlled microwave exposure heats the inside.
U.S. Pat. No. 4,230,924 discloses a food package including a flexible wrapping sheet of dielectric material, such as polyester or paperboard, capable of conforming to at least a portion of the article of food's shape. The dielectric wrapping sheet has a flexible metallic coating thereon, such as aluminum, in the form of a relatively thin film or relatively thick foil subdivided into a number of individual metallic islands or pads separated by criss-crossing non-metallic gaps provided by exposed dielectric strips on the wrapping sheet. When the food package is placed in a microwave oven, some of the microwave energy passes through the wrapping sheet so as to dielectrically heat the food item, but a lesser amount of the microwave energy is converted into thermal energy by the metallic coating so as to brown or crispen that portion of the food adjacent thereto.
U.S. Pat. No. 4,267,240 discloses a food package of plastic film or other dielectric substrate having a very thin coating thereon which absorbs the microwave energy when the package is placed within a conventional microwave oven. The plastic film or other substrate and its coating conform to a substantial surface portion of the food item. The coated plastic film or other substrate converts some of the microwave energy into heat which is transmitted directly to the surface portion of the food so that a browning and/or crispening is achieved.
In the case of susceptors employing thin metal coatings on a polymeric substrate, only a certain amount of heating can be achieved with a given metal before it begins to arc and destroy the polymeric sheet as well as the metallic coating itself. Arcing may also adversely affect the oven, and in some circumstances may even result in a fire.
For metallized films, there is a certain metal thickness that results in maximum microwave heating without arcing. This optimum metal thickness depends on a number of factors such as the nature of the metal itself, the uniformity of the metallic coating, the amount of oxygen and other impurities in the coating, and surface characteristics of the film substrate. Due to these factors, arcing may occur at the film thickness required for optimum heating. Thus, the arcing phenomenon may limit the amount of heat capable of being safely generated from a given single layer construction.